1. Field of the Invention
This invention relates to an environmentally cleaner method of purifying caprolactam.
2. Prior Art
Known from the pertinent literature are methods of purifying caprolactam from:
(a) its solutions in H.sub.2 SO.sub.4 following Beckmann transposition of cyclohexanoneoxyme, or
(b) solutions in H.sub.2 SO.sub.4 of the product resulting from nitrosation of cyclohexyl compounds and extraction of the unreacted cyclohexyl compound.
Such prior methods are characterized by the following process steps:
(1) neutralization of the sulphuric acid with NH.sub.3 to form the so-called "oil of lactam", which stratifies over the saturated solution of ammonium sulphate;
(2) causticization of the lactam oil with NaOH (in order to salify impurities of an acidic nature);
(3) extraction with toluene (or another equivalent solvent) of the caprolactam from the caustic oil;
(4) a range of appropriate treatments, such as chemical treatments, re-extraction with water of the caprolactam from the toluene solutions, additional chemical treatments, and finally vaporizations and/or rectifications.
Step (2), namely causticization with NaOH, has two main disadvantages:
(a) disposal of the toluene-extracted caprolactam raffinate: this water-alkaline stream contains
the sodium salts of the carboxylic acid impurities, PA1 the sodium salts of the organic carboxy-sulphonic impurities.
The only possible disposal technique is combustion of the whole stream; such combustion, however, is not devoid of some serious drawbacks, since the alkalis and chlorides present will corrode and attack the metal parts, as well as the refractories of the furnaces; further, during the combustion, there are formed meltable salts which, on the one side, tend to clog up the furnaces resulting in frequent stops, and on the other side, cause additional problems for their removal on account of their being present in large amounts (on the order of thousands of tons per year in the instance of a caprolactam plant having an 80,000 t/year capacity);
(b) poor solubility of the sodium salts of carboxysulphonic acids in causticized lactam oil, thereby causing precipitation of said salts in the extraction towers with toluene, which leads to frequent clogging, entrainments, poor separation of the phases, and accordingly, more or less long downtime for the plant.
The applicants have now unexpectedly found that such drawbacks can be fully obviated, where
(1) the caprolactam oil is alkalized, rather than treated with NaOH, with ammonia to a pH in the 8 to 10 range;
(2) any caprolactam left in the toluene solution is treated with NaOH.